Transmitting constellation symbols in a communication system

ABSTRACT

A method is provided for transmitting constellation symbols in a communication system. A set of orthogonal time-frequency mapping patterns (TFPs) is generated from a TFP. And a TFP from the set of orthogonal TFPs is selected for transmitting a block of constellation symbols.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 13/403,753, filed on Feb. 23, 2012, which is acontinuation application of U.S. patent application Ser. No. 11/653,114,filed on Jan. 11, 2007, which is a continuation of InternationalApplication PCT/CN2004/000128, filed on Feb. 17, 2004. Theafore-mentioned patent applications are hereby incorporated by referencein their entireties.

TECHNICAL FIELD

The present disclosure is related to the field of radio communications,and in particular to radio communication systems employingtime-frequency mapping pattern.

RELATED TECHNOLOGY

In a communication system including several users sharing thetransmission medium, i.e. the available communication resources,attention is given to the co-existence of the different signals beingpresent within the communication system. The users of the communicationsystem generally share the same pool of communication resources. Whenallocating the communication resources, e.g. different channels, tomultiple users, it is realised that the signal of one user may affect orinterfere with the signal of another user. A communications systemdesigner thus has to design a user traffic multiplexing scheme bearingthis in mind, and thus design the multiplexing scheme so as to handlethis undesired interference.

In communication systems in which a geographical division is used, i.e.a cellular system, there are mainly two kinds of multi-user interferencepresent. Firstly, the interference from users within the samegeographical area, e.g. a cell, the so called intra-cell interference,and secondly the interference from users in adjacent or neighbouringcells, the so called inter-cell interference. The inter-cellinterference may be decreased for example by means of resource planning,e.g. frequency planning, so that a specific communication resource isreused in such a way that interference is minimised. For frequencyplanning the inter-cell interference may be minimised by using afrequency reuse scheme, in which a certain frequency is not used inneighbouring cells. However, resource planning, for example frequencyplanning and coordination between cells, is time consuming, expensiveand in some cases not even feasible.

Besides resource planning, whereby inter-cell interference may bedecreased, there are other ways to decrease interference. One way todecrease both intra-cell interference and inter-cell interference is toutilise frequency hopping. Frequency hopping consists in changing thefrequency used by a channel at regular intervals. Thus, cells using thesame frequencies but different, presumably de-correlated, hoppingsequences lead to decreased interference.

WO2003/001696 describes a method for decreasing inter-cell interference.Frequencies are allocated to cells in a communication system accordingto functions selected to minimise repeated collisions between hoppingsequences used by the base stations of neighbouring cells. This is thusan example of a prior art method for decreasing inter-cell interferencein a communication system, and in which system also resource planning isperformed.

SUMMARY

In one aspect, an embodiment of the present disclosure provides a methodof generating and allocating time-frequency mapping pattern (TFP) in acommunication system. The method comprises: generating a TFP, generatinga set of orthogonal TFPs from said TFP; and allocating a TFP from theset of orthogonal TFPs to a user equipment in a transmission timeinterval (TTI) within a cell of the communication system.

In accordance with one embodiment of the present disclosure the generictime-frequency pattern is a generic Costas sequence. Such Costassequence based time-frequency patterns have desirable properties withrespect to interference and diversity, providing high diversity gainwhile at the same time minimising inter-cell and avoiding intra-cellinterference. Further, all T-F patterns in the set are obtained from afirst pattern giving an easily implemented and easily administrated wayto obtain orthogonal T-F mapping patterns.

In accordance with one embodiment of the present disclosure said Costassequence is obtained by a T4 construction. This choice of Costassequence provides improved diversity gain compared to the other choicesof Costas sequences. In accordance with one embodiment of the presentdisclosure a random cyclic offset is changed for each transmission timeinterval (TTI), according to a cell-specific pseudo-random sequence.Consequently, the different cells within the communication system willuse in principle different cyclically shifted versions of the same setof time-frequency mapping patterns. The unique cross-correlationproperties of the set of time-frequency mapping patterns ensure limitedcross-interference between any two cells at any time. This randomoffsetting in each TTI also makes instantaneous interference to appearnoise-like.

In accordance with another embodiment of the present disclosure said setof orthogonal T-F mapping patterns is generated by cyclic shifts in thefrequency domain of said generic time-frequency (T-F) mapping pattern(TFPgeneric). Thereby a set of orthogonal T-F mapping patterns isobtained in an easy and convenient fashion, and further ensures that theset of patterns is orthogonal. A random variable cyclic offsetting couldthen be performed in the time domain, giving maximum cross-correlationsnot significantly higher than the ideal values guaranteed for Costassequences by definition.

In an alternative embodiment said set of orthogonal T-F mapping patternsis generated by cyclic shifts in the time domain, and a random variablecyclic offsetting could then be performed in the frequency domain.Thereby a very flexible solution for multiplexing is provided, giving anetwork designer alternative ways to implement the present disclosure.

In accordance with another embodiment of the present disclosure theorthogonal T-F mapping patterns are randomly allocated to multiple usersand/or traffic channels in each TTI. This feature decreases theprobability of collisions between signals from the different cells.

In accordance with another embodiment of the present disclosure, atransmitter, a base station, a user equipment for performing said methodis provided, and such a system, both yielding corresponding advantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically the structure of the communication resourcesin a time-frequency divided communication system.

FIG. 2 shows an exemplary time-frequency mapping pattern.

FIG. 3 shows another exemplary time-frequency mapping pattern.

FIG. 4 shows an exemplary system.

DETAILED DESCRIPTION OF EMBODIMENTS

A brief description of an OFDM (Orthogonal Frequency DivisionMultiplexing) system is provided. However, an OFDM system is only anexample of a time-frequency divided system in which the presentdisclosure may be implemented, and it is understood that the disclosuremay be implemented in other time-frequency divided systems as well.

OFDM is a transmission technique that allows high data rates to betransmitted over very noisy channels, yet at a comparatively lowcomplexity, and is used for digital audio broadcasting (DAB) and digitalvideo broadcasting (DVB). OFDM has several favourable properties likehigh spectral efficiency and robustness to channel dispersion, for whichreasons it will most likely be used for future broadband applicationssuch as digital mobile radio communication.

In an OFDM system the data to be transmitted is spread over a largenumber of carriers, and the data rate to be transferred by each of thesecarriers is consequently reduced in proportion to the number ofcarriers. The carriers have an equal, precisely chosen frequencyspacing, and the frequency bands of the sub-carriers are not separatebut overlap. By using an IFFT (Inverse Fast Fourier Transform) asmodulation, the spacing of the sub-carriers is chosen in such a way thatat the frequency, where a received signal is evaluated, all othersignals are zero. The choice of carrier spacing is made so thatorthogonality is preserved, giving the method its name.

OFDM systems transmit constellation symbols block-wise. A block ofconstellation symbols is transmitted during one OFDM symbol interval.During a subsequent OFDM symbol interval, a new block of constellationsymbols is transmitted and so on. Thus, any transmitted constellationsymbol in an OFDM system can be characterised by two indexes: the firstindex indicating during which OFDM symbol interval it is transmitted,and the second index indicating which of the sub-channels it istransmitted on.

With reference to FIG. 1, a time-frequency resource grid is shown, whereTTI (Transmission Time Interval) indices are shown on the x-axis, andsub-channel indices on the y-axis. In the illustrative communicationsystem used to explain an embodiment of the present disclosure, data istransmitted in packets and each packet is transmitted during atransmission time interval, TTI. A TTI consists of a fixed finite numberof OFDM symbol intervals. Each cell in the figure can carry aconstellation symbol and is characterized by the two indexes mentionedabove: the first index indicating during which particular OFDM symbolinterval in a particular TTI it is transmitted and the second indexindicating which particular sub-carrier frequency (sub-channel index) isused for its transmission.

User traffic multiplexing is the allocation of transmitter resources(such as time, frequency, antennas, etc.) to the different trafficchannels within the same cell, so that the resulting physical channelscan co-exist preferably without mutual interference, or with as littleinterference as possible. Particularly, in an embodiment, it is assumedthat the communication resources to be allocated are time and frequencydivided resources, divided into time slots (TS) and frequency sub-bands,respectively, where each sub-band contains a number of subcarriers. Thenthe user traffic multiplexing can be defined as the allocation of aparticular sequence of sub-bands for the transmission of each trafficchannel during a TTI.

A time-frequency mapping pattern (TFP) is a sequence of indices of thesub-bands used for transmission within a TTI. The time-frequency mappingpatterns thus specify the different physical channels or trafficchannels, one T-F mapping pattern for each physical channel or eachtraffic channel.

An OFDM unit is a group of constellation symbols transmitted in asub-band during a time slot (OFDM symbol interval). Thus a TFP is usedto map a number of OFDM units onto the time-frequency grid within a TTI.In practice, certain sub-carriers will be reserved for pilots andsignalling, which may lead to variation in OFDM unit size and to mappingof certain OFDM units onto non-contiguous sub-carriers.

In accordance with an embodiment of the disclosure, all the cells of acellular communication system employ the same, special set of T-Fmapping patterns for multiplexing the traffic. Thus no network resourceplanning is needed, and consequently no network capacity needs to bereserved for such planning. Thus, given the fact that other neighbouringcells employ the same resource grid (users in neighbouring cells thusrun the risk of transmitting data on the same sub-carriers and duringthe same OFDM symbol intervals), the problem is now to assign thecommunication resources in each cell to the users in such a way that

1. The interference within cells is minimised (intra-cell interference),

2. The interference form other cells is minimised (inter-cellinterference), and

3. The diversity-performance for each user is maximised.

For a given user, the inter-cell interference appears in the form of“hits” or “collisions” (i.e. occurrences of data in other cellstransmitted at the same frequencies and during the same OFDM symbolinterval), either from an identical TFP used in another cell or fromanother TFP used in another cell.

In order to obtain a maximised diversity gain, the design of TFP shouldbe such that every pair of OFDM units is separated in time and frequencyas much as possible. This qualitative description can be mathematicallyformulated as the requirement to maximise the minimum Lee distancebetween the elements of a TFP. The Lee distance between the two pointsis the sum of the absolute values of the differences of thecorresponding coordinates.

In accordance with an embodiment of the present disclosure, the genericT-F mapping pattern TFPgeneric, might be obtained from a Costassequence. Briefly, a Costas sequence is a mathematical sequence havingcertain particularly beneficial correlation characteristics. Bydefinition, the number of hits between a Costas sequence and itsarbitrary (non-cyclic) time and frequency-shifted version is equal to 0or 1. For further information on Costas sequences, see for example S. W.Golomb and H. Taylor, “Construction and properties of Costas arrays”,Proc. IEEE, vol. 72, pp 1143-1163, September 1984.

The inventors of the present disclosure have realised the beneficialproperties of Costas sequences for application to a multiplexing schemein an OFDM system. When basing the TFPs used in a communication systemon the Costas sequence, all the above mentioned aspects of resourceallocation is optimised concurrently. That is, the inter-cellinterference is minimised, the intra-cell interference is avoided, thediversity performance for each user is maximised and no network planningis needed. There are mathematical functions that may render an evenbetter diversity performance, but then the interference performancewould suffer.

In accordance with an embodiment of the disclosure, a orthogonal set ofTFPs is obtained from a generic Costas sequence. This may beaccomplished either by using the original-size Costas sequence or byadjusting it to the size of the T-F grid, as will be explained more indetail below by means of some specific examples. The adjustment mayconsist of a periodic extension of the generic Costas sequence or ofshortening the generic Costas sequence.

In accordance with an embodiment of the disclosure, an orthogonal set ofTFPs is obtained by cyclic shifts in the frequency domain of the genericTFP, and thereby there will be no interference within a cell. In analternative embodiment the orthogonal set of TFPs is obtained by cyclicshifts in the time domain of the genericTFP. This design ensures thatthe set of TFPs is orthogonal, i.e. no hits (or collisions) will occurbetween the different patterns. Further, all the availabletime-frequency resources are utilised, meaning that all time slots (i.e.OFDM symbol intervals) and all sub-bands are used if all patterns aredeployed.

High diversity gain of each TFP is achieved, since the generic Costassequence possesses good Lee distance properties. The diversity gain ofall TFPs can be further maximised by choosing a specially constructedCostas sequence, having an improved minimum Lee distance.

With reference now to FIG. 2, starting with the shown exemplary T-Fmapping pattern, 14 cyclic shifts in the frequency domain may beperformed, giving a total of 15 TFPs. This set of TFPs is an orthogonalset and is allocated to a single cell. All these TFPs are thencyclically time-shifted by a cell specific offset, corresponding to aninteger number of OFDM symbols. That is, a time slot index (e.g. aninteger in the interval 1-15) may be randomly assigned to a cell. Thiscell specific time offset is changed for each TTI in accordance with acell-specific pseudo-random sequence. In that way the different cells,even though being synchronous in one specific TTI, which causesinterference, they will most likely be asynchronous in the next TTI. Thecross-interference is thus minimised, as predicted by the correlationproperties of the TFPs. Stated another way, the inter-cell interferenceis randomised in time, from one transmission time interval to another,by the random cyclic time-shifts of the whole set of orthogonal codes.

In an alternative embodiment, the 14 cyclic shifts are not performed inthe frequency domain, but in the time domain, again giving a total of 15TFPs. This set of TFPs is also an orthogonal set and is allocated to asingle cell. All these TFPs are then cyclically frequency-shifted(instead of time-shifted) by a cell specific offset, corresponding to aninteger number of OFDM symbols. That is, a frequency sub-band index (forexample an integer in the interval 1-15) may be randomly assigned to acell. This cell specific offsetting is again changed for each TTI inaccordance with a cell-specific pseudo-random sequence. As in theembodiment above, the different cells, even though being synchronous inone specific TTI (thus causing interference), they will most likely beasynchronous in the next TTI, and the cross-interference is thusminimised.

The different sequences of offsets, both in time or in frequency, canfor example be generated as time-shifted versions of a singlepseudo-random sequence.

In accordance with an embodiment of the present disclosure, one TFP ormore TFPs may be allocated to a single user, for example in dependenceon the amount of traffic data for the transmission, or in dependence onavailable communication resources or on the priority of a specific user.However, within each cell, no TFP is assigned to more than one userduring the same TTI, and thereby any potential intra-cell interferenceis eliminated.

In accordance with an embodiment of the present disclosure, randomallocation of the orthogonal TFPs to a plurality of users and/or trafficchannels in each TTI, decreases the probability of collisions betweenthe signals from the different cells. It is however contemplated thatthe allocation may be performed in a pseudo-random or even non-randomway, i.e. in a fixed way.

In spite of the design of the generic TFP as a Costas sequence and inspite of the fact that the sets of TFPs in neighbouring cells aresubject to a random cyclic time-shift, occasionally the exact same TFPmay appear in two neighbouring cells at the same time. In particular,when random allocation of the orthogonal TFPs to a plurality of usersand/or traffic channels in each TTI is performed, the properties of theCostas sequences are best exploited and the probability that all OFDMunits collide is reduces significantly, compared to other fixedallocation strategies.

As the set of TFPs in some TTI is obtained by cyclically shifting thegeneric TFP in the frequency or time domain, the property definitionmentioned earlier (i.e. number of hits between a Costas sequence and itsarbitrary (non-cyclically) time and frequency-shifted version equals 0or 1) is not applicable exactly and the number of hits may be higher.However, it is reasonable to expect, and can in fact be proven byexamples, that the actual maximum cross-correlations are notsignificantly higher than the ideal values.

An embodiment of the disclosure is described below by means of specificexamples of the allocation in accordance with the inventive method.

EXAMPLES

Again with reference to FIG. 2, the set of 15 orthogonal TFPs, one foreach OFDM physical channel or traffic channel, is derived from a singleCostas sequence of length 15 obtained from the so-called T4construction. This generic TFP, TFPgeneric, is shown in FIG. 2, as asequence of indices of the frequency sub-bands used for transmissionwithin one TTI.

Case (A): For a T-F grid with 12 OFDM symbol intervals (time slots) and15 frequency sub-bands, the first pattern is obtained by discarding thelast three symbols of the generic Costas sequence, in order to obtainthe patterns of length NOFDM=12.

Case (B): For a T-F grid with 27 OFDM symbol intervals (time slots) and15 frequency sub-bands, the first pattern is obtained by extending thegeneric Costas sequence by the reversed first 12 symbols of the samegeneric pattern, in order to obtain the patterns of length NOFDM=27.

Mathematically these two cases may be expressed as

TFP0(A)=TFPgeneric(1:12)

TFP0(B)=[TFPgeneric TFPgeneric(12:-1:1)]

where (a:b) denotes the sequence of integers (a, a+1, a+2, . . . , b−1,b), (b:−1:a) denotes the sequence of integers (b, b−1, b−2, . . . , a+1,a), and [A B] denotes the straightforward concatenation of 2 sequences.

For the first case, case (A), the first two T-F mapping patterns aregiven by:

TFP0(A)=[13 5 3 9 2 14 11 15 4 12 7 10]

TFP1(A)=[14 6 4 10 3 15 12 1 5 13 8 11]

where TFP0(A) maybe regarded as a generic TFP. TFP1(A) is obtained bycyclic shift of TFP0(A).

For the second case, case (B), the first two T-F mapping patterns aregiven by:

TFP0(B)=[13 5 3 9 2 14 11 15 4 12 7 10 1 6 8 10 7 12 4 15 11 14 2 9 3 513]

TFP1(B)=[14 6 4 10 3 15 12 1 5 13 8 11 2 7 9 11 8 13 5 16 12 15 3 10 4 614]

where TFP0(B) maybe regarded as a generic TFP. TFP1(B) is obtained bycyclic shift of TFP0(B).

With reference to FIG. 3, another generic Costas sequence, with minimumLee distance equal to 3, is shown. In particular, the Costas sequenceshown is a T4 Costas sequence of length 27. The T4 construction ensuresTFPs with a minimum Lee distance equal to 3. The Lee distance gives anindication of the proximity of elements of aTFP, as was explained above,and this choice of Costas sequence separates every pair of OFDM units asmuch as possible in time and frequency, and thus gives a very highdiversity gain.

Now with reference to FIG. 4, an embodiment of the present disclosurealso encompasses a communication system, generally denoted 100,implementing the inventive method. The communication system 100 ispreferably divided into several cells 110. In each cell, a base station120 serves an user equipments 130. The base station 120 and userequipment 130, respectively, includes transmitter(s) including means forimplementing the method in accordance with the present disclosure. Aperson skilled in the art realises that the user traffic multiplexingmay be performed elsewhere in the system, such as for example in amobile switching centre (MSC), a base station controller (BSC), or thelike, depending on the communication system in question. The basestation and user equipment 130, respectively, may for example include atransceiver and processor (not shown) appropriately programmed forwireless communication in accordance with the invented method formultiplexing. It is further understood that the demultiplexing isperformed correspondingly.

In summary, an embodiment of the present disclosure provides a methodyielding all of the following advantages:

1. Intra-cell interference is avoided.

2. Inter-cell interference is reduced.

3. Most of the available diversity in the TTI is captured.

4. The above advantages 1-3 are accomplished without network resourceplanning.

The disclosure has been described in conjunction with embodiments. It isevident that numerous alternatives, modifications, variations and useswill be obvious to a person skilled in the art in light of the foregoingdescription. For example, the communication system need not be an OFDMsystem, so the disclosure could also be used in other frequency-hoppingsystems, such as for example GSM systems.

1. A method of transmitting constellation symbols in a communicationsystem, comprising: obtaining, by a user equipment (UE), a firsttime-frequency mapping pattern (TFP), wherein the first TFP is selectedfrom a set of orthogonal TFPs, the set of orthogonal TFPs is generatedby: cyclically shifting a second TFP in frequency domain, and cyclicallyshifting the second TFP in time domain; and transmitting, by the UE, ablock of constellation symbols in accordance with the first TFP.
 2. Themethod of claim 1, wherein the first TFP is a sequence of indices ofsub-bands used for transmission within a transmission time interval(TTI).
 3. The method of claim 2, wherein the TTI comprising at least onetime slot.
 4. The method of claim 2, wherein each of the sub-bandscomprises a number of subcarriers.
 5. The method of claim 1, wherein thefirst TFP is configured to map the block of constellation symbols onto atime-frequency grid within a TTI.
 6. The method of claim 5, wherein theblock of constellation symbols is grouped into at least one orthogonalfrequency-division multiplexing (OFDM) symbol.
 7. The method of claim 5,wherein the TTI comprises a number of OFDM symbol intervals.
 8. Themethod of claim 7, wherein each of the number of OFDM symbol intervalsis configured to transmit a plurality of constellation symbols.
 9. Themethod of claim 5, wherein the time-frequency grid is represented by adimension of time slot and a dimension of sub-band.
 10. The method ofclaim 1, wherein the first TFP is randomly selected from the set oforthogonal TFPs.
 11. A user equipment in a communication system,comprising a transceiver and a processor, wherein the processor isconfigured to: obtain a first time-frequency mapping pattern (TFP),wherein the first TFP is selected from a set of orthogonal TFPs, the setof orthogonal TFPs is generated by: cyclically shifting a second TFP infrequency domain, and cyclically shifting the second TFP in time domain;and transmit a block of constellation symbols in accordance with thefirst TFP.
 12. The user equipment of claim 11, wherein the first TFP isa sequence of indices of sub-bands used for transmission within atransmission time interval (TTI).
 13. The user equipment of claim 12,wherein the TTI comprising at least one time slot.
 14. The userequipment of claim 12, wherein each of the sub-bands comprises a numberof subcarriers.
 15. The user equipment of claim 11, wherein the firstTFP is used to map the block of constellation symbols onto atime-frequency grid within a TTI.
 16. The user equipment of claim 15,wherein the block of constellation symbols is grouped into at least oneorthogonal frequency-division multiplexing (OFDM) symbol.
 17. The userequipment of claim 15, wherein the TTI comprises a number of OFDM symbolintervals.
 18. The user equipment of claim 17, wherein each of thenumber of OFDM symbol intervals is configured to transmit a plurality ofconstellation symbols.
 19. The user equipment of claim 15, wherein thetime-frequency grid is represented by a dimension of time slot and adimension of sub-band.
 20. The user equipment of claim 11, wherein thefirst TFP is randomly selected from the set of orthogonal TFPs.
 21. Amethod of transmitting constellation symbols to a base station (BS) in acommunication system, comprising: obtaining, by a user equipment (UE),communication resources allocated by the BS; obtaining, by the UE, afirst time-frequency mapping pattern (TFP), wherein the first TFP isselected from a set of orthogonal TFPs, the set of orthogonal TFPs isgenerated by: cyclically shifting a second TFP in frequency domain, andcyclically shifting the second TFP in time domain; and transmitting, bythe UE, a block of constellation symbols to the BS on the communicationresources in accordance with the first TFP.